The present invention relates to a method of orienting the magnetic film of a magnetic disc, which is used in a magnetic disc memory device, by a magnetic field, and more particularly to a method of orienting the magnetic film of a magnetic disc by a magnetic field in such a manner that the surface of the magnetic film is made smooth and the magnetic squareness of the magnetic film is improved.
In order to achieve the large memory capacity required for a recent magnetic disc memory device, it is desirable to record a large number of data bits on a magnetic disc at a high recording density. In order to realize the high density recording, it is necessary to form a thin, uniform magnetic layer on the magnetic disc and to make the surface of the magnetic layer smooth. The thin magnetic layer has an advantage such that a signal reproduced from the magnetic layer has a high resolution, but has a disadvantage such that the output level of the reproduced signal is lowered. In order to prevent the lowering in the output level of the reproduced signal, it is necessary to improve the orientation ratio of magnetic particles contained in the magnetic layer. The magnetic layer of a magnetic disc is formed of a resin which contains magnetic particles having magnetic poles. The above-mentioned lowering in the output level of the reproduced signal can be prevented by orienting almost all of the magnetic particles in a desired direction. The technique for orienting magnetic particles on a magnetic disc in a desired direction is called "particle orientation by magnetic field", and it is expressed by the orientation ratio how many magnetic particles on the magnetic disc have been oriented in the desired direction.
The particle orientation for a magnetic disc is carried out in the fabrication process of the magnetic disc, and is usually performed immediately after a thin magnetic film (that is, a magnetic layer) is formed on the surface of the magnetic disc. This is because since, immediately after the thin magnetic film has been formed, a solvent contained in the thin magnetic film has not been evaporated yet with the film having a low viscosity, magnetic particles can be readily oriented in a desired direction by an applied magnetic field, and the solvent is more and more evaporated with a lapse of time with the film having a higher viscosity to finally fix the orientation of each magnetic particle.
Now, conventional methods of orienting the magnetic film of a magnetic disc will be explained below, by reference to FIGS. 3A, 3B, 4A and 4B. In these figures, reference numeral 1 designates a magnetic disc in a state immediately after a thin magnetic film has been formed on the magnetic disc, and 3a, 3b, 4a and 4b magnets.
FIG. 3A is a plan view for explaining a conventional method of orienting the magnetic film of a magnetic disc, and FIG. 3B is a sectional view taken along the line IIIB--IIIB of FIG. 3A. Referring to FIGS. 3A and 3B, a pair of magnets 3a and 3b and another pair of magnets 4a and 4b are disposed on the side of one surface of the magnetic disc 1, and a thin magnetic film 1a is provided on the other surface, so that a horizontal magnetic field is applied to the magnetic disc almost in its circumferential direction by the magnets polarized in a direction perpendicular to the disc surface. When the magnetic disc is rotated in the above state, magnetic particles in the thin magnetic film are oriented substantially in the circumferential direction of the disc. However, since the horizontal magnetic field is applied to the magnetic disc by the magnets disposed on the rear side of the disc, the magnetic particles are actuated only vertically at a portion of the film on the disc right under the magnets, as shown in FIG. 3B. Accordingly, it is impossible to improve the unevenness roughness of the surface of the thin magnetic film sufficiently. Further, a considerable number of defects may be left within the thin magnetic film. In order to solve the above problems, another method of orienting the magnetic film of a magnetic disc has been proposed. FIG. 4A is a plan view for explaining this method, and FIG. 4B is a sectional view taken along the line IVB--IVB of FIG. 4A. Referring to FIGS. 4A and 4B, a pair of magnets 3a and 3b are disposed on the upper side of the magnetic disc 1 so as to have the polarity configuration shown in these figures, and another pair of magnets 4a and 4b are disposed on the lower side of the magnetic disc 1 so as to have the polarity configuration shown in the figures. The magnetic disc 1 is rotated with a horizontal magnetic field applied to the disc from each magnet pair in the circumferential direction. When the magnetic disc is rotated under the above-mentioned conditions, a horizontal magnetic field due to the magnets 3a and 3b and another horizontal magnetic field due to the magnets 4a and 4b are alternately applied to each part of a thin magnetic film 1a formed just now on the magnetic disc 1, and thus magnetic particles in the magnetic film 1a. are oriented in the circumferential direction of the magnetic disc 1. At the same time, the magnetic particles are moved in upward and downward directions alternately, and thus are uniformly distributed in the thin magnetic film 1a. In the above method, however, two kinds of horizontal magnetic fields are alternately applied to the thin magnetic film, and hence the orientation efficiency of the magnetic film is not high. Thus, the magnetic squareness of the magnetic film would be about 0.8 at most, even if the number of magnets used were increased or the polarity of some of the magnets 3a, 3b, 4a and 4b were reversed. Further, in a different method of orienting the magnetic film of a magnetic disc by arranging a pair of magnets in such a manner that the magnetic disc is interposed between the magnets and similar magnetic poles of the magnets face each other (that is, a repulsive force is generated between the magnets), the magnetic squareness of the magnetic film can be increased to about 0.9, but magnetic particles in the magnetic film are gathered and nonuniformly, distributed so that the aluminum substrate of the magnetic disc is locally exposed, that is, the nonuniformity of the magnetic film is caused by the applied magnetic field. The above method is disclosed in Japanese patent unexamined publication No. 58-130444.